Correspondence  |   March 2012
Why the Gray Zone May Shift within the Fog
Author Affiliations & Notes
  • Joshua A. Bloomstone, M.D.
  • *Banner Thunderbird Medical Center, Glendale, Arizona, and Valley Anesthesiology Consultants, Phoenix, Arizona.
Article Information
Correspondence   |   March 2012
Why the Gray Zone May Shift within the Fog
Anesthesiology 3 2012, Vol.116, 739-740. doi:10.1097/ALN.0b013e3182472339
Anesthesiology 3 2012, Vol.116, 739-740. doi:10.1097/ALN.0b013e3182472339
To the Editor: 
We read the article by Cannesson1 et al.  and the accompanying editorial2 with great interest, and praise the effort to better define the clinical utility and applicability of pulse pressure variation (PPV), not only as a tool to predict volume responsiveness but also to move away from a single-threshold value for conducting intraoperative volume optimization and perioperative goal-directed fluid therapy. We would like to make three points.
First, the most commonly used index to assess volume responsiveness, by far in the United States, appears to be stroke volume variation (SVV). Although the area under the receiver-operating curve, in systematic reviews,3 shows that both PPV and SVV have excellent sensitivity and specificity in patients who are mechanically ventilated with normal tidal volumes and in a regular sinus rhythm, the threshold values discriminating between fluid responders and nonresponders are not the same for these two parameters.3,4 Thus, although strategies using the “gray zone approach” applied to PPV identify a range of values where volume responsiveness cannot reliably be predicted, this range may not be applicable when SVV is used for determining volume responsiveness. Although it is clear from the work of Joseph Erlanger, the father of the pulse pressure concept,5 that pulse pressure in man is proportional to left ventricular stroke volume (depending both on arterial tone and cardiac contractility) it is plausible that variations in pulse pressure may also be proportional to variations in stroke volume. To the extent that PPV and SVV are affected differently by changes in arterial tone, given the same degree of volume responsiveness,4 these values may lose their direct proportionality as vascular tone changes. Pinsky described the potential utility of a SVV-to-PPV ratio to reflect ventricular-arterial coupling that might be helpful when extrapolating PPV threshold data to SVV.6 Overall, applying PPV's “gray zone” to SVV seems clinically appealing but may be misleading. The “gray zone” defined for PPV should not simply  be applied to patients who are being optimized using stroke volume variation. The “gray zone” for SVV requires its own definition.
Second, the paper does not address the issue of volume responsiveness versus  needing to give volume or using a dynamic index to actively restrict fluids or to administer diuretics to patients under certain clinical circumstances. That is, not all patients who are volume responsive require volume therapy. Conversely, there may be untapped utility for SVV- and/or PPV-guided fluid restriction and diuretic use (consider patients with acute lung injury or acute respiratory distress syndrome).7 The goal would be to identify patients with a PPV that is “too low” who, consequently, should receive restrictive fluid therapy or diuretics.
Third, what is, perhaps, not considered by the author2 is the fact that PPV goals may vary within individual patients across changing clinical settings in relatively short intervals of time, such as during single lung ventilation for thoracic surgery, laparoscopic pneumoperitoneum, and conditions of pathologic intraabdominal hypertension. For example, the goals for intraoperative PPV during esophagectomy vary during the case, such that the goal during the abdominal part of the procedure is fluid liberal and during the thoracic part of the procedure is fluid restrictive.8 
In summary, we praise Cannesson et al.  1 for their work and we agree that there is a “gray zone” well represented by the picture in the editorial2 (i.e.  , the Golden Gate Bridge with fog going across the middle represents “a static view of dynamic indices”). We contend that PPV and SVV “gray zones” are probably better described as the view one gets of the Golden Gate Bridge, with fog going across the middle as one is driving along winding hilly roads. This is a “dynamic view of dynamic indices,” such that the gray zone changes at different times depending on one's changing vantage point, an analogy closer to clinical reality.
Cannesson M, Le Manach Y, Hofer CK, Goarin JP, Lehot JJ, Vallet B, Tavernier B: Assessing the diagnostic accuracy of pulse pressure variations for the prediction of fluid responsiveness: A “gray zone” approach. ANESTHESIOLOGY 2011; 115:231–41
De Hert SG: Assessment of Fluid Responsiveness Insights in a “gray zone.” ANESTHESIOLOGY 2011; 115:229–30
Marik PE, Cavallazzi R, Vasu T, Hirani A: Dynamic changes in arterial waveform derived variables and fluid responsiveness in mechanically ventilated patients: A systematic review of the literature. Crit Care Med 2009; 37:2642–7
Pinsky MR: Hemodynamic evaluation and monitoring in the ICU. Chest 2007; 132:2020–9
Erlanger J, Hooker D: An experimental study of blood pressure and pulse pressure in man. Johns Hopkins Hospital Records 1904; 12:145–378
Pinsky M. (2006). Protocolized cardiovascular management based on ventricular-arterial coupling. In: Jean-Louis Vincent (Ed.), Functional Hemodynamic Monitoring, Update In Intensive Care Medicine (pp. 381–95). Springer-Verlag Berlin Heidelberg
Mailloux PT, McGee WT, Nathanson B: Hemodynamic and delta blood volume relationship during continuous renal replacement therapy. Abstracts 2009; 136:50S–e
Mena GE, Raghunathan K, McGee WT: Intraoperative monitoring. In: Principles of Practice of Anesthesia for Thoracic Surgery. Springer Science and Business Media, LLC, New York. 2011, pp. 265–76